As was mentioned above the present invention has turned out to give considerable advantages in connection with liquid phase hydrogenations, and in view of that the invention will primarily be described and exemplified in connection therewith. However, the idea of the invention is not in any way restricted to this type of reaction but should be generally applicable to chemical reactions of the kind referred to.
Liquid phase hydrogenations are very commonly used processes within the chemical industry, from very large petrochemical plants to small apparatuses of the type that is utilized inter alia within the pharmaceutical industry. In the manufacture of so-called fine or performance chemicals of a complex nature the synthesis method often comprises a large number of stages. Obviously it is of great importance that the yield of each stage will be a maximum yield to have the possibility of obtaining a total yield which is economically competitive. Therefore, the selective action of the catalyst will be decisive in a liquid phase hydrogenation as well as in every other step of the total synthesis. In this connection the problems with reference to liquid phase hydrogenations can be schematically illustrated by means of the following simple example.
A compound B containing a hydroxyl amine group is to be prepared from the corresponding nitro compound A. Through the fact that the desorption of B from the catalyst surface is relatively slow time permits B to be hydrogenated further to the corresponding amine compound C, and in view of that the yield of B will be low. To varying extents this is applicable to many consecutive, catalytic reactions. Variations of hydrogen pressures and temperatures have very small influences upon the yield, which means that the hydrogenation step must be supplemented with an extensive and in some cases energy consuming processing step. From this example it is apparent that already very small improvements as to catalyst efficiency can mean important contribution to and savings in connection with the technique within this field.
An improvement of the technique in connection with catalytical reactions is disclosed in Swedish Lay-Open Print No. 7114167-5. The technique disclosed therein is based on an improvement of the non-wettability of the catalyst carrier, i.e. carbon, by means of at maximum the same amount of polytetrafluoroethylene as of the catalyst carrier. According to said Swedish Lay-Open Print the manufacture of the catalyst particles having improved non-wettability properties is accomplished by mixing said catalyst particles with polytetrafluoroethylene in the form of an emulsion, the water then being evaporated by means of heat to the formation of a pure mixture of catalyst particles and polytetrafluoroethylene. The catalyst particles having improved non-wettability properties can then be utilized in a bed or shaped to an element and give some improvements of the yield as compared to particles which have not been treated in this way to obtain non-wettability.
As to catalysts for e.g., gas-liquid reactions reference can also be made to Derwents Abstracts No. 73066B/40, DE-A-2835943, EP-A1-066 676, EP-A1-085 350, DE-B2-2104019 and U.S. Pat. No. 4,259,209 but neither of these documents discloses or suggests the combination of features which are essential to the present invention as described more in detail below. On the contrary the most relevant of said documents are based on wet methods comprising impregnation with a solution of a precursor of the final catalyst in a solvent, e.g. chloroplatinic acid in acetone, which obviously means a more or less uncontrolled penetration of the catalyst into the porous material and no continuous layer on the surface of the porous carrier but rather some kind of "islands" of the catalyst particles.